What Are SIM Cards?by Stephen Lilley
A SIM card should be familiar to anyone who has ever had to purchase a new cell phone. For those who haven't, a SIM card is a tiny chip that fits inside your phone. It contains any important information you store in your phone during the duration of your ownership, and much more.
SIM is short for Subscriber Identity Module. A SIM card is a small removable chip that fits inside your phone. It contains a unique code that identifies you on your cellular provider's network. This is done so the network can differentiate each of its countless users.
Every SIM card contains what is called an IMSI, or International Mobile Subscriber Identity. When the phone is on it is connected to the network of the cellular provider. This enables the provider to separate one user from the other. It is this number that also allows a cellular provider to aid in police investigations since each phone appears on the network differently.
SIM cards also store a number of specific bits of information used to differentiate users from each other on a network. These include the operator-specific emergency number, the service provider name and the local area identity.
Calls are actually connected not by their phone numbers, but by their International Mobile Subscriber Identity numbers. This is a special number that only the network can see. It is stored within the SIM card. A user's phone sends a signal to the network requesting access to a specific IMSI, at which point the signal is sent to the requested phone.
SIM cards also store user-specified data. A phone's text-based messages and address book contacts are stored on the SIM card. This is convenient because if you're upgrading to a new phone you can simply take the SIM card out of your old phone and put it in your new one and instantly have access to all of this information.
- photo_camera Motorola.com